A power supply, e.g. a power supply for a light bulb assembly, should exhibit short start-up times, in order to allow for a quick start-up of the device coupled to the power supply, e.g. in order to allow for a quick start-up of the light bulb assembly. The power supply may be configured to convert electrical energy from a mains supply to electrical energy for a light source of a light bulb assembly at a pre-determined voltage level. Examples for light sources are light emitting diodes (LED) and/or organic LEDs (OLEDs), which are typically referred to as solid state lighting (SSL) devices. The power supply may also be referred to as a driver circuit.
Light bulb assemblies may be dimmable subject to control events received via the mains supply. Typically, phase-cut dimmers are used to cut a phase of the waveform of the current provided by the mains supply to the light bulb assembly. The phase-cut angle is typically indicative of the degree of dimming to be provided by the light bulb assembly. In the present document, a method for enabling dimming without the need of a phase-cut dimmer is described. This method makes use of brief interruptions of the mains supply which may be created by a brief off/on cycle using a mains power switch. Such a brief interruption of the mains supply may be interpreted by a controller comprised within the driver circuit of the light bulb assembly as an instruction to dim the light.
It is expected that the dimming of SSL based light bulb assemblies in response to events triggered by such off/on cycles of the mains power switch will become an important topic in the near future, as this technique allows the implementation of dimming without the need for a phase-cut dimmer. In order to enable a light bulb assembly to detect such off/on cycles, the controller of the driver circuit of the light bulb assembly may be provided with a counter for measuring the length of an OFF period, i.e. the period during which the mains supply is off. By doing this, the controller can distinguish between an off/on cycle (having a pre-determined maximum duration of the OFF period) for triggering the dimming of the light bulb assembly, and an intentional “turn-off” of the light bulb assembly (if the duration of the OFF period exceeds the pre-determined maximum duration).
If a timer or counter within the controller is used to measure the duration of an OFF period, the controller needs to maintain active operation even during time windows with no mains supply. As a consequence, relatively large storage capacitors may be required, in order to keep the controller running during the OFF period. However, the use of additional storage capacitors adds costs and space to the driver circuit. Furthermore, the use of additional and/or large storage capacitors may have a negative impact on the lifetime of the light bulb assembly, because such storage capacitors are typically Aluminum Electrolyte type capacitors which exhibit reduced lifetimes. In addition, the use of large storage capacitors (e.g. large supply voltage capacitors) may have a negative impact on the start-up time of the driver circuit and of the light bulb assembly.
The present document addresses the above mentioned technical problems and describes a driver circuit and/or power supply which enables the use of relatively small supply voltage capacitors (for short start-up times, reduced costs, reduced space and increased lifetime), while at the same time enabling the determination of the duration of an OFF period (for providing a light bulb assembly which is dimmable subject to off/on events of the mains supply).